Using an iRule for SMTP Relay and Exchange 2013

There is a difference between the irule you posted and what is reported in the TCL error.

Nonetheless, It appears that smtp_relay_allowed is supposed to be the name of a data-group defined via the GUI. If so, use it without the ''$::' and without surrounding it by quotes.

when CLIENT_ACCEPTED { set accepted_snat "IP Address"

if { [ class exists smtp_relay_allowed ] } { 
    if { [class match [IP::client_addr] equals smtp_relay_allowed] } { 
       snat $accepted_snat 
    } else { 
       snat automap
    }
} else { 
    snat automap 
}
}